Scott T. Allen

Scott T. Allen
University of Nevada, Reno | UNR · Department of Natural Resources and Environmental Science

Ph.D

About

72
Publications
38,904
Reads
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1,457
Citations
Introduction
Scott Allen is an assistant professor at University of Nevada, Reno.
Additional affiliations
October 2020 - present
University of Nevada, Reno
Position
  • Professor (Assistant)
May 2019 - September 2020
University of Utah
Position
  • PostDoc Position
February 2017 - May 2019
ETH Zurich
Position
  • PostDoc Position

Publications

Publications (72)
Article
Hydrologic connectivity provisions water and nutrient subsidies to floodplain wetlands, and may be particularly important in floodplains with seasonal water deficits through its effects on soil moisture. In this study, we measured sapflow in 26 trees of two dominant floodplain forest species (Celtis laevigata and Quercus lyrata) at two hydrological...
Article
Full-text available
Stand production, a common indicator of site productivity, is often low in flooded wetland forests. However, it remains unclear whether the low production reflects flood stress limiting tree growth, flood disturbance limiting tree density, or both. Here, we investigate how tree growth and stand density vary in baldcypress forests, across gradients...
Article
Full-text available
Quantifying seasonal variations in precipitation δ2H and δ18O is important for many stable isotope applications, including inferring plant water sources and streamflow ages. Our objective is to develop a data product that concisely quantifies the seasonality of stable isotope ratios in precipitation. We fit sine curves defined by amplitude, phase,...
Article
Full-text available
Stable isotope tracers often vary between different parts of the water cycle. Such data are crucial for refining our understanding because they reveal that water moves through the environment in ways that are not always captured by hydrometric approaches and that are at odds with many commonly-used hydrological models. For example, isotopic composi...
Article
Full-text available
Rain recharges soil water storages and either percolates downward into aquifers and streams or is returned to the atmosphere through evapotranspiration. Although it is commonly assumed that summer rainfall recharges plant-available water during the growing season, the seasonal origins of water used by plants have not been systematically explored. W...
Article
Full-text available
The National Ecological Observatory Network (NEON) provides open-access measurements of stable isotope ratios in atmospheric water vapor (δ ² H, δ ¹⁸ O) and carbon dioxide (δ ¹³ C) at different tower heights, as well as aggregated biweekly precipitation samples (δ ² H, δ ¹⁸ O) across the United States. These measurements were used to create the NEO...
Article
Full-text available
Plain Language Summary In the middle of a hot and dry summer, we often think that a large rain event can “rescue” a forest from drought. However, it is not clear whether trees can or do consistently use the water from summer rains. A growing body of research indicates that over the course of the summer growing season, trees take up significant amou...
Preprint
Full-text available
To predict hydrologic responses to inputs and perturbations, it is important to understand how precipitation is stored in catchments, released back to the atmosphere via evapotranspiration (ET), or transported to aquifers and streams. We investigated this partitioning of precipitation using stable isotopes of water (2H and 18O) at the Can Vila catc...
Article
Full-text available
The stable isotopes of hydrogen and oxygen in xylem water are often used to investigate tree water sources. But this traditional approach does not acknowledge the contribution of water stored in the phloem to transpiration, and how this may affect xylem water and source water interpretations. Additionally, there is a prevailing assumption that ther...
Article
Full-text available
A wide range of hydrological, ecological, environmental, and forensic science applications rely on predictive “isoscape” maps to provide estimates of the hydrogen or oxygen isotopic compositions of environmental water sources. Many water isoscapes have been developed, but few studies have produced isoscapes specifically representing groundwaters. N...
Chapter
Full-text available
The water present within trees when sugars and cellulose are formed is the source of hydrogen and oxygen atoms that are incorporated into tree-ring cellulose (see Chaps. 10.1007/978-3-030-92698-4_10 and 10.1007/978-3-030-92698-4_11 ). However, the isotope composition of relevant water pools is often unknown when trying to interpret δ ¹⁸ O and δ ² H...
Chapter
Full-text available
The hydrogen isotopic signature (δ ² H) of water in trees contains information on plant functional responses to climatic changes and on the origin of the water. This is also true for the non-exchangeable hydrogen isotopic signature (δ ² H NE ) of plant organic matter, which contains additional physiological and biochemical information that can be d...
Article
This is the first Europe-wide comprehensive assessment of the climatological and physiological information recorded by hydrogen isotope ratios in tree-ring cellulose (δ²Hc) based on a unique collection of annually resolved 100-year tree-ring records of two genera (Pinus and Quercus) from 17 sites (36°N to 68°N). We observed that the high-frequency...
Preprint
Full-text available
The degree of water mixing in the critical zone is under intense debate. Field measurements of isotope ratios indicate varying degrees of separation between pools of water that supply streams and vegetation. The exact physical mechanisms behind ecohydrologic separation are unknown, but local conditions such as soil heterogeneities likely influence...
Preprint
Full-text available
Groundwater is an invaluable global resource, but its long-term viability as a resource for consumption, agriculture, and ecosystems depends on precipitation recharging aquifers (Alley et al., 2002; Gleeson et al., 2012). How much precipitation recharges groundwaters varies enormously across Earth's surface (Scanlon et al., 2006; Moeck et al., 2020...
Article
Full-text available
The interception of precipitation by plant canopies can alter the amount and spatial distribution of water inputs to ecosystems. We asked whether canopy interception could locally augment water inputs to shrubs by their crowns funneling (freshwater) precipitation as stemflow to their bases, in a wetland where relict overstory trees are dying and pe...
Article
Full-text available
Plant transpiration links physiological responses of vegetation to water supply and demand with hydrological, energy, and carbon budgets at the land–atmosphere interface. However, despite being the main land evaporative flux at the global scale, transpiration and its response to environmental drivers are currently not well constrained by observatio...
Article
Sampling intervals of precipitation geochemistry measurements are often coarser than those required by fine-scale hydrometeorological models. This study presents a statistical method to temporally downscale geochemical tracer signals in precipitation so that they can be used in high-resolution, tracer-enabled applications. In this method, we separa...
Article
Full-text available
There are scientific gray areas where it is unclear whether process representations and assumptions in models have adequate empirical basis. Nonetheless, useful insights can come from interpreting data with the aid of models that use hypothetical parameterizations or process representations, even if they are uncertain and speculative. Indeed, virtu...
Article
Full-text available
Carbon fluxes in terrestrial ecosystems and their response to environmental change are a major source of uncertainty in the modern carbon cycle. The National Ecological Observatory Network (NEON) presents the opportunity to merge eddy covariance (EC)‐derived fluxes with CO2 isotope ratio measurements to gain insights into carbon cycle processes. Co...
Preprint
Full-text available
Recent studies have demonstrated that plant and soilwater extraction techniques can introduce biases and uncertainties in stable isotope analyses. Here we show how recently documented δ2H biases resulting from cryogenic vacuum distillation of water from xylem tissues may have influenced the conclusions of five previous studies, including ours, that...
Article
Although typically a strong determinant of site quality, the influence of flooding is not well understood for mixed-species floodplain forests. It is, however, generally assumed that growth is limited in floodplains that experience stagnant flooding because they are disconnected from river channels. Here, this assumption was tested across three con...
Article
Full-text available
Plant transpiration links physiological responses of vegetation to water supply and demand with hydrological,energy and carbon budgets at the land-atmosphere interface. However, despite being the main land evaporative flux at the global scale, transpiration and its response to environmental drivers are currently not well constrained by observations...
Article
Salinity intrusion is responsible for changes to freshwater wetland watersheds globally, but little is known about how wetland water budgets might be influenced by small increments in salinity. We studied a forested wetland in South Carolina, USA, and installed sap flow probes on 72 trees/shrubs along a salinity gradient. Species investigated inclu...
Article
Full-text available
Stable isotope ratios of water (𝛿18O,𝛿2H) have long been used to study a core question in plant ecology and ecohydrology: “From where do plants take up water?” Indeed, decades of research has involved sampling potential plant water sources in the subsurface, classifying those sources as distinct endmembers (e.g., deep versus shallow soil waters), a...
Article
Full-text available
The “Ecohydrologic separation” hypothesis challenged assumptions of translatory flow through the rooting zone. However, studies aiming to test ecohydrologic separation have largely done so with insufficient discussion of infiltration and rooting zone recharge processes, and instead have mostly focused on either isotopic differences between stream w...
Article
Full-text available
Hurricanes can cause immediate catastrophic destruction of marsh vegetation and erosion of soils; however, they also have long-lasting ecological impacts. Those impacts include the deposition of tremendous amounts of saltmarsh litter (‘wrack’) onto upland ecosystems, the hydrologic effects of which have not previously been investigated. When Hurric...
Article
Full-text available
A portion of precipitation drains to the surface down plant stems, as “stemflow.” Although per observations to date, stemflow rarely represents >2% of gross precipitation in forests, it can result in larger water fluxes to near-stem soils that are hypothetically more important to roots. The ecohydrological importance of stemflow is often predicated...
Article
Full-text available
Forests play a key part in the water cycle, so both planting and removing forests can affect streamflow. In a recent Article1, Evaristo and McDonnell used a gradient-boosted-tree model to conclude that streamflow response to forest removal is predominantly controlled by the potential water storage in the landscape, and that removing the world’s for...
Article
Full-text available
The terrestrial water cycle partitions precipitation between its two ultimate fates: “green water” that is evaporated or transpired back to the atmosphere, and “blue water” that is discharged to stream channels. Measuring this partitioning is difficult, particularly on seasonal timescales. End-member mixing analysis has been widely used to quantify...
Chapter
The interception of precipitation by vegetation has important consequences for climate and water resources. Although canopy interception has been studied for centuries, many fundamental unknowns remain. We present persistent questions that reflect challenges in measuring, representing, and understanding how terrestrial ecosystems intercept, partiti...
Article
Full-text available
The stable water isotopes, 2H and 18O, can be useful environmental tracers for quantifying snow contributions to streams and aquifers, but characterizing the isotopic signatures of bulk snowpacks is challenging because they can be highly variable across the catchment landscape. In this study, we investigate one major source of isotopic heterogeneit...
Article
Full-text available
The time that water takes to travel through the terrestrial hydrological cycle and the critical zone is of great interest in Earth system sciences with broad implications for water quality and quantity. Most water age studies to date have focused on individual compartments (or sub-disciplines) of the hydrological cycle such as the unsaturated or sa...
Article
Full-text available
Quantifying the relative contributions of winter versus summer precipitation to streamflow may be important for understanding water‐resource sensitivity to precipitation variability. Here we compare volume‐weighted mean δ18O values in precipitation and streamflow for 12 catchments in Switzerland, to determine whether summer or winter precipitation...
Article
Full-text available
A key attribute of the terrestrial water cycle is the partitioning of precipitation into its two ultimate fates: green water that is evaporated or transpired back to the atmosphere, and blue water that is discharged to stream channels. Measuring this partitioning is difficult, particularly on seasonal timescales. End-member mixing analysis has been...
Preprint
Full-text available
Forests play a key role in the water cycle, so both planting and removing forests can affect streamflow. In a recent Nature article, Evaristo and McDonnell used a gradient-boosted-tree model to conclude that streamflow response to forest removal is predominantly controlled by the potential water storage in the landscape, and that removing the world...
Article
Full-text available
This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through on-line media, followed by two workshops through which a large number of potential science questions were collated, p...
Article
Forested wetlands play a vital role in the coastal zone, but their vulnerability to coastal change-and the impact to high-value ecosystem services-is not as well established as that of more seaward systems such as saltmarsh and mangroves. To address this need, we develop field-based stand density classes, then classify baldcypress-water tupelo (Tax...
Article
Full-text available
River flooding is a common hazard, causing billions of dollars in annual losses. Flood impacts are shaped by the spatial scale over which different rivers flood simultaneously, but this dimension of flood risk remains largely unknown. Using annual flood data from several thousand European rivers, we demonstrate that the flood synchrony scale—the di...
Preprint
Full-text available
Forested wetlands play a vital role in the coastal zone, but their vulnerability to drivers of coastal change-and the impact to high-value ecosystem services-is not as well established as that of more seaward systems such as saltmarsh and mangroves. To address this need, we develop field-based stand density classes, then classify baldcypress-water...
Article
Full-text available
River flooding is a common hazard, causing billions of dollars in annual losses. Flood impacts are shaped by the spatial scale over which different rivers flood simultaneously, but this dimension of flood risk remains largely unknown. Using annual flood data from several thousand European rivers, we demonstrate that the flood synchrony scale – the...
Presentation
Full-text available
The time water takes to travel through the hydrological cycle is of great interest in earth system sciences because water travel times reflect how water flows through landscapes, with implications for water quality and quantity. To date, most water age studies have focused on the individual compartments of the water cycle such as the unsaturated an...
Poster
Full-text available
Many catchment hydrology studies use stable water isotopes as conservative tracers to track the relative contributions of recent precipitation (event water) in streamflow and to infer dominant runoff processes. Here we present two alternative approaches for using stable water isotopes to better understand how catchments store and release water: est...
Article
Studies of stable isotopes of water in the environment have been fundamental to advancing our understanding of how water moves through the soil-plant-atmosphere continuum; however, much of this research focuses on how water isotopes vary in time, rather than in space. We examined the spatial variation in the δ18O and δ2H of throughfall and bulk soi...
Article
Full-text available
Rain recharges soil water storages and either percolates downward into aquifers and streams, or is returned to the atmosphere through evapotranspiration. Although it is commonly assumed that summer rainfall recharges plant-available water during the growing season, the seasonal origins of water used by plants have not been systematically explored....
Article
Studies of stable isotopes of water in the environment have been fundamental to advancing our understanding of how water moves through the soil‐plant‐atmosphere continuum; however, much of this research focuses on how water isotopes vary in time, rather than in space. We examined the spatial variation in the δ18O and δ2H of throughfall and bulk soi...
Article
Full-text available
In this commentary, we summarize and build upon discussions that emerged during the workshop "Isotope-based studies of water partitioning and plant-soil interactions in forested and agricultural environments" held in San Casciano in Val di Pesa, Italy, in September 2017. Quantifying and understanding how water cycles through the Earth's critical zo...
Article
The sharing of data and collection of new data are both essential, but they are not inherently complementary. When data are openly available, researchers may be motivated to use those data rather than collect more because field work has costs and risks. The competitive advantage to those who do not put resources towards fieldwork may discourage fie...
Article
Full-text available
The young water fraction Fyw, defined as the proportion of catchment outflow younger than approximately 2–3 months, can be estimated directly from the amplitudes of seasonal cycles of stable water isotopes in precipitation and streamflow. Thus, Fyw may be a useful metric in catchment inter-comparison studies that investigate landscape and hydro-cli...
Article
Full-text available
In this commentary, we build on discussions that emerged during the workshop "Isotope-based studies of water partitioning and plant-soil interactions in forested and agricultural environments" held in San Casciano Val di Pesa, Italy, in September 2017. Quantifying and understanding how water cycles through the Earth's critical zone is important to...
Article
Understanding how precipitation isotopes vary spatially and temporally is important for tracer applications. We tested how well month‐to‐month variations in precipitation δ18O and δ2H were captured by sinusoidal cycles, and how well spatial variations in these seasonal cycles could be predicted, across Switzerland. Sine functions representing seaso...
Conference Paper
Full-text available
Comparing catchments' hydrologic responses to their physiographic properties and climatic forcing helps to identify dominant controls on water storage and streamflow generation mechanisms. Here, we used the young water fraction (Fyw), i.e. the fraction of streamflow younger than roughly 3 months, for a catchment inter-comparison study across 22 sit...
Article
Full-text available
The young water fraction Fyw, defined as the proportion of catchment outflow younger than ca. 2-3 months, can be estimated directly from the amplitudes of seasonal cycles of stable water isotopes in precipitation and streamflow. Thus, Fyw may be a useful metric in catchment inter-comparison studies that investigate landscape and hydro-climatic cont...
Article
Growth responses of wetland trees to flooding and climate variations are a critical link in hydrological control of wetland carbon budgets, but they are difficult to model because they depend on multiple interacting and nonlinear factors. To more generally understand hydrological influence on tree growth, we modeled non-linear responses of tree rin...
Article
The isotopic composition of water transmitted by the canopy as throughfall or stemflow reflects a suite of processes modifying rainfall. Factors that affect isotopic composition of canopy water include fractionation, exchange between liquid and vapor, and selective transmittance of temporally varying rainfall along varying canopy flowpaths. Despite...
Article
Decreased water availability due to hydrologic modifications, groundwater withdrawal, and climate change threaten bottomland hardwood (BLH) forest communities. We used satellite-derived (MODIS) land-surface temperature (LST) data to investigate spatial heterogeneity of canopy temperature (an indicator of plant-water status) in a floodplain forest o...
Article
Full-text available
Among previous studies, there are large discrepancies in the difference between evapotranspiration from wetland vegetation and evaporation from open water. In this study, we investigate evapotranspiration differences between water and vegetation in a scenario that has otherwise not been extensively investigated: evapotranspiration from floodwaters...